The present invention relates generally to virtual display optical systems, and more particularly, to a wide spectral bandwidth virtual image display optical system.
The prior art for virtual image display systems, such as wide spectral bandwidth head-up displays (HUD) is generally classified into two categories that include (1) systems using one combiner and an all-refractive relay, or (2) systems using one combiner and a convex reflective relay. In case (1), the secondary axial, lateral and high order chromatic aberrations severely limit the usable spectral bandwidth, eye box (pupil) size and field of view. In case (2), the generic telephoto optical form of this optical system design limits its usable field of view.
A typical conventional head-up display (HUD) or helmet-mounted display (HMD) is described in U.S. Pat. No. 3,940,204. The optical system disclosed in this patent includes three sub-modules: one holographic combiner, one relay group and one cathode my tube (CRT). The information displayed on the CRT is first relayed by a relay group to form an intermediate image and further relayed by the combiner to a comfortable position in front of a viewer. This optical design form has the following advantages. (1) The relay group allows the CRT size to be small. (2) The relay group enlarges the area of the overlay field. (3) The combiner relays the eye box (pupil) inside the relay group. Therefore, the size of the relay group is relatively small.
However, one drawback of this type of optical system is that the usable spectral bandwidth associated with the holographic combiner is very narrow. One way to correct this deficiency is to replace the holographic combiner with a reflective combiner. A second drawback associated with this type of helmet mounted display system is that the secondary axial and lateral chromatic aberration caused by the refractive relay group limits its usable spectral bandwidth.
Special glass materials such as KZFSN4 and FK52, and the like, manufactured by Schott, for example, are often used to correct secondary chromatic aberration associated with long effective focal length (EFL) optical modules. However, a typical head-up display and helmet mounted display require a low F-number, large field of view and long EFL. Therefore, higher order chromatic aberrations such as primary spherochromatism, chromatic coma and chromatic distortion, and secondary spherochromatism are present. Chromatic coma and chromatic distortion often severely degrade the image quality. In addition, the special glass materials are very expensive and are difficult to fabricate.
Furthermore, commercial and military head-up displays and automobile head-up display are normally used with monochromatic or quasi-monochromatic image generators. It appears that the majority of future display systems will desire the use of color displays. However, there is no simple conventional design form that will satisfy this requirement.
It is therefore an objective of the present invention to overcome the deficiencies inherent in conventional wide spectral bandwidth virtual image display optical systems. It is a further objective of the present invention to provide a virtual image display optical system that may be used with color image generators.